1. Field of Invention
This invention relates to a structure of a photodiode image sensor device. More particularly, the present invention relates to a structure of a CMOS image sensor (CIS).
2. Description of Related Art
A photodiode image sensor device is the most commonly used device for detecting images. A typical photodiode image sensor device comprises a reset transistor and a light sensor region formed by a photodiode. For example, a photodiode is formed with an N type doped region and a P type substrate. When the photodiode image sensor is in operation, a voltage is applied to the reset transistor gate to turn on the reset transistor and to charge the N/P diode junction capacitor. The reset transistor is turned off when the charging of the N/P diode junction capacitor has reached a certain high voltage. The N/P diode generates a reverse bias to form a depletion region. When a light is shinned on the N/P diode light sensor, electrons and holes are generated. These holes and electrons are separated by the electrical field of the depletion region, causing the electrons to travel in the direction of the N-type doped region to lower the voltage of the N-type doped region, whereas the holes travel in the direction of the P-type substrate.
A charge coupled device (CCD) has a high dynamic range and a low dark current. The sophistication of the current technology of a charge coupled device allows the charged couple device to become the most popular image sensing device. The manufacturing for a charge coupled device is, however, rather special. The price for a CCD is therefore very high. Moreover, the driver requires a high voltage operation, leading to the problems of high power dissipation and inability of random access of memory.
A CMOS image sensor has the characteristics of high quantum efficiency, low read noise, high dynamic range and random access, and it is one hundred percent compatible with the manufacturing for a CMOS device. A CMOS image sensor can combine with other control circuit, A/D converter and several signal processing circuits on a single wafer to achieve the so-called system on a chip (SOC). The progress of the technology of a CMOS image sensor, therefore, greatly reduces the cost of an image sensor device, the picture size and the power of dissipation. The CMOS image sensor is therefore slowly replacing the charge coupled device.
The structure of a conventional CMOS image sensor is summarized in the following.
Referring to FIG. 1A, a field oxide layer 102 is formed on a substrate 100. A reset transistor 120 that comprises a gate oxide layer 104 and a polysilicon gate 106 is formed on the substrate 100. The source/drain region 108 and the doped region 112 of the photodiode sensor region 110 are formed by implanting and thermal driving-in ions using the field oxide layer 102 and the polysilicon gate 106 as masks. A spacer 114 is formed on the sidewalls of the polysilicon gate 106 and the gate oxide layer 104. A self-aligned block (SAB) 116 is further formed on the photodiode sensor region 110 to complete the formation for a CMOS image sensor device.
The conventional CMOS image sensor, however, has the following problems.
After the above CMOS image sensor is formed, the backend process is conducted, such as the formation of the inter-layer dielectrics and metal conductive line, which are used for the controlling of the device. The application of plasma etching is inevitable in the backend process for, for example, the defining of the contact/via opening or the metal conductive line. The high power plasma, however, can penetrate the inter-layer dielectrics to induce damages on the surface of the photodiode. The damages inflicted upon the surface of the photodiode due to plasma etching are especially prominent in the vicinity of the bird's peak region. As a consequence, current leakage occurs more easily in the photodiode sensor region. The aforementioned current leakage problem would cause the CMOS image sensor to generate a significant dark current, leading to an increase of read noise.